Coating compositions based on compounds with epoxy groups have a limited temperature resistance, and decomposition occurs above 160° C.
In U.S. Pat. No. 5,198,515, a process was described for modifying resins having olefinically unsaturated bonds by combining with bismaleimides. According to this method polymers having a heat resistance up to 500° C. can be obtained using 1,3-bis(citraconimidomethyl)benzene. This product is available under the trade names Flexlink® and Perkalink®.
Flexlink® has the chemical formula shown below, wherein D stands for 1,3-dimethylenebenzene and R is a methyl group:

The crosslink accelerator of this curing system is peroxide. However, for polymers having an epoxide group that are not cured in the presence of peroxide but in the presence of an amine-functional compound, only low amounts of Flexlink® can be used. The Flexlink® content in such curing system is below about 20%, due to its limited solubility. At higher concentrations Flexlink® forms sediment in the curing system.
In U.S. Pat. No. 4,837,295, epoxy resin adhesives have been disclosed in which polyoxyalkylene polyamines are used as curing agents and mono- or bis-unsaturated maleimides are present, but N,N′-hexamethylenedicitraconimide, for which no protection is sought, is also described. The maleimides of U.S. Pat. No. 4,837,295, however, are toxic and carcinogenic compounds, and moreover have different reactivity in comparison with the biscitranonimides of this disclosure. A further disadvantage of these compounds is that they cannot be used in low temperature applications because they are not soluble and solidify in the curing systems at room temperature.
The synthesis of biscitraconimide compounds having both ether and sulfone groups is disclosed in JP 01-186865, such as 4,4′-bis(3-citraconimidophenoxy)diphenyl sulfone. Such compounds are not used in the present disclosure.
There is a need for peroxide-free curing systems having much higher amounts of bismaleimides than possible with Flexlink®. It is therefore an objective of the invention to provide such systems which are less toxic and carcinogenic than the known systems.